5 Nov 1929. 1st Lt Edwin R. Page, Chief of the U.S. Army Air Corps (hereinafter USAAC) Materiel Division Engineering Section Power Plant Branch, wrote Engineering Section Chief Maj C.W. Howard, regarding design and procurement of 1,000 hp engines. This is the first known official correspondence regarding the USSRC liquid-cooled engine doctrine. His memorandum states:

1. It has been appreciated for sometime past that European engines were producing considerably higher performances than those built in this country. It is believed that American engines are mechanically equivalent, if not better, than anything in the world today, and the remarkable performances exemplified by such equipment as the R-6 engine in the Schneider Cup Race recently, are largely due improvements in fuel and design changes in the equipment made possible by the use of such fuels. (AUTHOR'S NOTE: 1st Lt Page is probably referring to the Rolls-Royce "R" engine that powered the Supermarine S.6 seaplane that won the Schneider Trophy during the 6 – 7 Sep 1929 races at Calshot, Hampshire, England.) These performances indicate that the Air Corps today is almost hopelessly behind England in the matter of engine performance, and if England were to cease all development work today, it would probably take the Air Corps eighteen (18) months to design and build equipment with equivalent performances. This is a serious matter and it is believed one of serious military importance.

2. Considering the above conditions, the Materiel Division has been working for sometime past on specifications of an engine which, it is hoped, can compete on even terms with British development. Specifications have been prepared at this division and considerable work on layouts and design has been done by the Curtiss Company, which look extremely promising.

3. It is proposed to build an engine of strictly service type, with no thought to racing, but an engine capable of several gear combinations for use in pursuit, attack, observation and bombing. From a standpoint of standardization, and especially in emergency conditions, this is believed to be the most efficient policy.

4. The proposed engine will be designed to develop 1,000 hp at 2,700 rpm and will be rated as such. The engine, however, will be capable of 3,300 rpm, with a corresponding increase in power. The weight will be approximately 1,000 lb. It will be Prestone cooled and, in general, will represent the highest development in conventional design possible.

5. It is believed that the type of engine most desirable at this time is not as large an engine as the Schneider Racer, but one of some 1,600 or 1,700 in³ displacement, and with a maximum output and fuel economy under these conditions. This results in an engine which is not so large as to render pursuit equipment unmaneuverable and to require enormous weights in fuel.

6. The general design of the proposed engine will be somewhat along lines of the present V-1570 (Curtiss Conqueror), for the reason that this engine represents the highest specific development in this country, and the new Rolls-Royce which was used in the Schneider Racer, is of identically the same type. With a view to conservatism and the hope of getting the best performance in the shortest time, it was decided to follow this conventional type with every possible refinement conceivable, based on the work and experiences of the various engine manufacturers, both here and in Europe, and experimentation carried on at the Division.

7. An opportunity was given to the various engine manufacturers to view and discuss the new specifications, with the result that the Curtiss Company, being the only manufacturer with large experience in this particular type, is the only one seriously interested, and furthermore, it is the organization equipped to build such an engine in the shortest time. Accordingly, the Curtiss Company, on their own initiative, made an exhaustive study, prepared layout drawings and some details. This work appears extremely promising, and if the project is approved, it is believed that the first engine will be available for test one (1) year from the date of contract. This, in itself, would be an outstanding achievement in the manufacture of a new engine.

[USNARA RG342 RD3285. 452.8 - Engines - 1000 Horsepower, 1929-30-31-32-33-34-36. (hereinafter Engines 1000 hp). 1-2.]

3 Dec 1932. PPL civilian employee Opie Chenoweth issued Memorandum Report M-57-83, "Development of High Output Aircraft Engines," which aptly summarized the USAAC liquid-cooled engine world. It is quoted in its entirety below:

1. Object: To outline the development of high output engines for use in the Air Corps.

2. Conclusions and Recommendations: In the development of high output engines for Air Corps use, there are certain component parts of the engine which require intensive development to obtain an engine which will be satisfactory for general Air Corps use. The parts which have been under investigation by the Air Corps are as follows:

(a) Internally cooled valves. The internally cooled valve has permitted higher compression ratios and higher degrees of supercharging than were possible prior to its invention. This type of valve is used in most of the high output engine, not only in this country, but in Europe as well, and was one of the features of the Rolls-Royce engine used in the last Schneider Cup Race. This valve was developed by Air Corps personnel.

(b) Supercharging. The development of a satisfactory supercharger is a prerequisite to obtaining extremely high engine outputs. The Air Corps has been actively engaged in the development of both geared centrifugal and exhaust driven turbosuperchargers. Engines having gear driven centrifugal superchargers similar to those being used on European engines at this time, have been used by the Air Corps for some years and in 1926 complete performance of a Curtiss D-12 engine with a geared centrifugal supercharger was obtained in the altitude chamber of the Bureau of Standards. At this early date, the geared centrifugal supercharger was used only as a means of maintaining sea level horsepower to altitude and engines were not then in the state of development which would permit ground boosting to any degree at sea level. Currently with the development of the geared centrifugal supercharger, activity has been carried on continuously with the exhaust gas driven supercharger. It is the opinion of the [Engineering] Division that this type of supercharger is imperative for obtaining sea level outputs at high altitudes of the order of 20,000 to 25,000 ft. While there have been a few exhaust gas driven superchargers built and tested in Europe, the project has not been actively undertaken. One point which should be emphasized in the consideration of the exhaust gas driven supercharger, is that without such a device a 1,000 hp engine at 20,000 ft develops approximately 800 hp, whereas an engine having an exhaust gas driven turbosupercharger installed, even though has a sea level rating of only 500 hp, would perform better at altitudes above 20,000 ft than the 1,000 hp sea level engine.

(c) Cooling. All of the extremely high output engine have been of the liquid cooled type. To obtain a cooling system for service type equipment, which does not present an excessive frontal area, some method of high temperature cooling is required to reduce not only the frontal area, but also the weight of the power plant installation. This situation does not, of course, exist on specialized racing airplanes wherein wing type radiators can be used, but this sort of an installation is not satisfactory for general military use. In the development of engines and cooling systems, which will operate at temperatures of 250 – 300°F, the Air Corps has contributed an important phase of development of high power engine installations for service type equipment, which will reduce power plant weight and frontal area.

Taking the recent Schneider Cup Race engines, as built by Rolls-Royce Company, as an example, it may be stated that these engines are a development of a standard service type of engine, the Model N, or Buzzard engine, rated at 935 hp at 2,300 rpm. The racing engines had the same bore and stroke as this service engine but were changed in many regards to obtain 2,300 hp at 3,300 rpm for the race. It is desired to point out that the type test for these special engines was of one hour duration, whereas the Air Corps requires its present type test  67.5 hrs of full power operation, and , in addition 82.5 hrs at part throttle operation. These engine are, therefore, in the same general class as the Packard engines used by Gar Wood in the recent Harmsworth [British Motorboat] Trophy. These were Packard engines, supercharged to a reported rating of 1,000 hp, whereas the basic engine was unsatisfactory for Air Corps use at 800 hp.

The Air Corps has been interested in the development of three 1,000 hp engine, which are the first steps torard the development of higher outputs.

(a) Allison GV-1710 Type Engine. The Air Corps has procured one of these 12 cylinder vee engines to be operated at 1,000 hp at 2,800 rpm, having a gear reduction of 2:1, and a maximum weight of 1,160 lb. This engine has passed satisfactory endurance tests at 750 hp and 2,400 rpm and has been modified to meet the Air Corps requirements by lengthening the reduction gear nose 12", reducing the overall height of the engine and changing the lower contour to obtain an ideal engine for installation in an airplane. In addition, many changes are being made in the supercharger, carburetion, valves, pistons, etc., to obtain higher output. This engine will obtain 1,000 hp at 168 psi bmep, which gives a considerable margin for development.

(b) Wright GSV-1800 Engine. The Air Corps has been closely watching the development of the Wright Aeronautical Corporation GSV-1800 engine, which is to be rated at 1,000 hp. It has a reduction gear ratio of 7:5 and an approximate weight of 1,160 lb. This engine will develop 1,000 hp at 175 psi bmep, which also leaves a considerable margin for future development.

(c) Pratt & Whitney GR-2060 Engine. The Air Corps is closely watching the development of the Pratt & Whitney 20 cylinder engine, arrange in pentagonal form, having 5 banks, 4 cylinders deep. This engine has a reduction gear ratio of 3:2 and is rated at 1,000 hp at 2,500 rpm at sea level and has a dry weight of 1,550 lb. This engine will develop 1,000 hp at 154 psi bmep, which leaves a large margin for development.

Since the Navy Department has been authorized to proceed with the development of a high speed seaplane for international competition, immediate development of an engine having a normal rating of 2,000 hp, which is estimated with further development could be increased to 3,000 hp, is being undertaken. As the first step, a 12 cylinder liquid-cooled hexagonal engine having 2,120 in³ displacement has been built and for a short period of time, 1,034 hp has been obtained at 2,415 rpm. Since this was at a 180 psi bmep, it would seem logical that 2,000 hp could readily be obtained from an engine having 24 of the same cylinders and a displacement of 4,240 in³ in the form of a hexagonal engine having banks 4 cylinders deep.

In concluding, it may be stated that there are no known practical service type engines operating at 1,000 hp anywhere in the world, and engines in excess of this output are constructed and utilized for special racing applications.

[3 Dec 1932 Memorandum Report M-57-83. Development of High Output Aircraft Engines. USNARA RG342 P010573]

Now that we have reviewed the background leading to the USAAC/Continental collaboration, an unvarnished examination of what the USAAC hoped to achieve, and how, is an interesting exercise. Included in this are comments by the author, who with the benefit of hindsight, illustrates that every single tenet of the USAAC doctrine was flawed:

1. Universal engine applicability to pursuit and bomber types? This pipe-dream of a single engine that can be outfitted with numerous propeller reduction gear types and installed in any military airplane persisted for decades, and still persists to this day for some gas turbine engines. I wish it were true, but it nearly never is. Engine designers should certainly follow established principles and use common cores, but the idea of one engine to rule them all is just nuts.
2. 1,000 hp? The 1,000 hp goal was OK for 1929, but development took far longer than anyone could have guessed. By the time a 1,000 hp engine passed a type test, 2,000 hp engines were needed. PPL engine development was always playing catch-up.
3. Many small, high-speed cylinders. This scheme resulted in impressive bmep values, but there is more to an aircraft engine than metrics. Once the cylinder count exceeded 12, the engine weight started to balloon. Worse, the high-speed cylinders did not breathe as well as larger, slower turning engines. Engine complexity increased, specific fuel consumption suffered, and the high-speed parts wore out sooner.
4. Prestone-cooled. PPL engine architects thought that by mandating engines cooled by pressurized ethylene glycol, a 300 °F coolant outlet temperature could be achieved, which would result in smaller coolant radiators that had less weight and cooling drag. The resulting cooling systems were more prone to leakage than ones using pure water. The higher coolant temperature raised the cylinder wall temperature, which severely impacted engine development. The pistons and ring packs were prone to excessive wear and early failure. However, the real rub was that the higher coolant temperature meant the oil coolers had to be commensurately larger, which removed most of the small-coolant-radiator advantage. Engine waste heat had to be dissipated, one way or another.
5. Separate cylinders. PPL engine experts thought that separate cylinders, as opposed to monobloc construction, would be required to ensure adequate coolant circulation; they also opined that separate cylinders would make maintenance easier. This philosophy resulted in the engines being longer and heavier, and the crankcases being weaker in bending and torsion than an equivalent monobloc engine. Every single USAAC high-output engine that featured separate-cylinder construction suffered from crankcase cracks and main bearing distress.
6. External altitude superchargers. Before WWI ended, the USAAS was enamored with Dr. Sanford A. Moss and the General Electric Company's turbosuperchargers<https://www.enginehistory.org/superchargers.shtml>. The appeal was that turbosuperchargers could provide sea-level pressure to engine induction systems essentially for free as the turbosuperchargers worked by extracting otherwise wasted engine exhaust energy to drive them. However, this intriguing concept had two flaws; the turbosupercharger turbines required exotic metal alloys to survive the heat and centrifugal stress, and the ducting necessary to make the turbosuperchargers work consumed an enormous volume. WWII experience would indicate the exotic metal supplies necessary to make turbosupercharger turbines were extremely limited, and were, in practice, only available to P-38 and P-47 fighters, and to B-17, B-24 and B-29 bombers. Turbosupercharger installation volume was evidenced by the Republic P-47, which was the SMALLEST airframe that could combine a 2,000 hp engine with a turbosupercharger installation. Other fighters and bombers had to use engine-driven superchargers, whose development the USAAC had neglected due to its focus on turbosuperchargers.

7. Buried Engine Patent

   Engine Configuration. Although the PPL envisioned an upright V-12, the Aircraft Laboratory, obsessed with minimal frontal area, insisted on very shallow-depth opposed engines that could be buried in fighter and bomber wings or very slim nacelles. While an attractive concept, actual practice showed that these buried engines competed with the real estate that wing spars, landing gears, fuel tanks and other components needed to occupy.
8. Obsession with Contra-Rotating and Two-Speed Propellers. The USAAC made all its contractors jump through these two hoops, spending countless hours arguing over requirements, standards and specifications. Then more countless hours were wasted designing and arguing over contra-rotating and two-speed propeller reduction gearboxes. No airplane with either technology ever entered service.

Additional Factors.

1. Insufficient development budget. USAAC-supported XO-1430 development mostly coincided with the Great Depression. The USAAC had too little money to support the development and Continental finances did not support private investment. This led to critical delays when the ONE single-cylinder test engine would break down, or when, for a long time the ONE 12-cylinder engine would break. Testing would sometimes stop for weeks while material was ordered and then fabricated into replacement parts.
2. USAAC Micromanagement. The Army treated the XO-1430 project as its own engine-development sandbox, changing its mind frequently about how to proceed and wasting the scarce development money. The USAAC was gleefully optimistic, an attitude that stifled pragmatism and diminished any sense of urgency.
3. The USAAC ignored engines that did not fit its paradigm. Hindsight shows that the Allison V-1710, which was there all along, ultimately would be the only liquid-cooled U.S. engine developed during the 1930s that would see production and be a success. In retrospect, USAAC's other development projects, including the XO-1430, soaked up a lot of money that could have been used to improve the Allison.